In a technical field of an optical communication, a communication capacity or a transmission distance are expanded. The communication capacity can be expanded by, for example, using a wavelength-division multiplexing (WDM) technique. The transmission distance can be expanded by using an optical amplifier such as an erbium-doped optical fiber amplifier (EDFA).
D1: JP 2001-86071 A
D2: WO 2010/107061
D3: JP 2010-124266 A
Since the optical amplifier is easy to generate noise light such as ASE (Amplified Spontaneous Emission) light, when signal light is amplified by the optical amplifier, an optical signal noise ratio (OSNR) of the signal light is easy to deteriorate.
Hence, when a plurality of optical amplifiers is applied to an optical transmission system to amplify and relay signal light at multiple stages, for example, the OSNR of the signal light may cumulatively deteriorates and may fall below a tolerance of an intermediate transmission section (which may also be referred to as a “span”) of the optical transmission system.
When the OSNR falls below the tolerance, even if light can be received, an optical receiver would be unavailable to accurately demodulate and identify a signal received by the light. Hence, there is a limit on expanding a transmission distance by using the optical amplifier.
In order to further expand a transmission distance, a regenerator may be applicable to the optical transmission system. The regenerator converts received signal light into an electrical signal, performs a signal regeneration processing such a signal waveform shaping of the electrical signal, converts the regenerated electrical signal into a signal light again, and transmits (or relays) the signal light. Thus, it is possible to improve the OSNR of the signal light and to expand the transmission distance of the signal light.
However, when the regenerator is applied to the optical transmission system, a cost of the optical transmission system may increase. For example, the regenerator available to regenerate and relay signal light with equal to or more than 100 Gbps is costly. Therefore, the optical transmission system introduced the regenerator would have a big cost impact.
Meanwhile, the OSNR of the signal light can be improved by increasing a gain of the optical amplifier to increase transmission optical power. However, in a WDM optical transmission system, transmission optical power of signal light per wavelength tends to decrease as the number of multiplexed wavelengths increases.
Hence, in order to expand a transmission distance of WDM light by increasing a gain of the optical amplifier without using the regenerator, it would be effective to increase pump light power of the optical amplifier according to the number of multiplexed wavelengths of WDM light.
The pump light power can be increased by increasing the number of pump light sources (e.g., semiconductor laser diodes (LDs)) or by applying a pump light source with high performance and high output power. However, these approaches would increase a cost of the optical amplifier, and therefore, a cost of the optical transmission system would also increase.